In the horn loudspeaker, the sound waves generated by the diaphragm are gathered at a throat of the horn so as to increase sound pressure, and thereafter, propagate along the horn. Aerial resistance in the horn exerts on the diaphragm so that radiation resistance is increased, thereby improving the transmission efficiency of the speaker.
The horn loudspeaker is advantageous in that it has a high energy conversion efficiency. Hence, it is applied for various purposes such as for a home hi-fi system and a commercial public address system.
The horn loudspeaker may be sorted into a parabolic horn speaker, conical horn speaker, exponential horn speaker and hyperbolic horn speaker, each of which differs from one another in increasing rate of the cross-sectional areas of the horn.
As an example of the horn loudspeaker, a conventional exponential horn loudspeaker is described with reference to FIGS. 8a and 8b which schematically shown a basic horn speaker and an equivalent circuit of the mechanical system of the speaker. The horn speaker has a horn having a throat at an apex thereof. For the ease of explanation, the length of the horn is considered to be infinite. Opposite to the throat is disposed a diaphragm connected to a converter for converting electrical energy to sound energy. In the figure, SO is an area(m.sup.2) of the diaphragm, S1 is an area(m.sup.2) of the throat, mo is a total mass(kg) of a diaphragm device, Cm is a compliance(m/n) of a suspension of the diaphragm, and Cm' is a compliance(m/n) of air in a space between the diaphragm and the throat.
An acoustic impedance Zr for unit area(Ns/m.sup.3) is expressed as EQU Zr=rr+jXr
where rr is a radiation resistance(Ns/m.sup.3) and Xr is a radiation reactance(Ns/m.sup.3) for unit area. The radiation resistance rr is further expressed as ##EQU1## where ZO is a specific acoustic impedance(Ns/m.sup.3) and mx is a flare constant of the horn. The radiation reactance Zr is expressed as EQU Xr=ZO(m/2k)
FIG. 9 shows a frequency response of the radiation impedance. The radiation resistance for unit area represents a rate of energy propagating from the throat to the mouth of the horn to the entire energy.
In the conventional horn speaker, there is a problem that for good sound reproduction at low frequencies, group delay characteristics are deteriorated, and vice versa.
FIG. 10 show a relaxation time .tau. relative to acoustic impedance. The relaxation time .tau. which is expressed as .tau..sub.2 Zr/rr shows a transient response of the horn. That is, when the relaxation time .tau. is increased, both the rise and fall of the frequency is delayed. In a low frequency range, the transient response becomes insufficient. Therefore, the sound in the transient state in the low frequency range is strengthened, thereby deadening the sound. As a result, the sound quality is deteriorated.
In addition, the sound source of the horn speaker is unstable. More particularly, a phase constant in the horn is expressed as ##EQU2## A wavelength .lambda.h(m) in the horn is expressed as ##EQU3## where fc is a cutoff frequency(Hz). When a given frequency f coincides with the cutoff frequency fc(f=fc), the wavelength .lambda.h is infinitely increased, so that the sound source moves.
FIG. 11 shows a ratio .lambda.h/.lambda. between the horn wavelength .lambda.h and the free space wavelength .lambda.. The graph indicates that the position of the sound source moves, particularly in a range where the frequency f is approximate the cutoff frequency fc.
Moreover, due to the relationship between a directional frequency response and energy characteristics, a peak appears in sound pressure frequency response, which means deterioration in smooth sound quality.